1. Field of the Invention
The present invention relates to processes for producing linear polyesters comprising a main structural unit of an aromatic dicarboxylic acid ester of an alkylene glycol and polyester elastomers comprising a polyalkylene terephthalate as a hard segment and a polyalkylene glycol as a soft segment.
2. Description of the Prior Art
The most important linear polyesters amongst those currently under industrial manufacture are polyalkylene terephthalates represented by polyethylene terephthalate and polytetramethylene terephthalate which have been utilized extensively for fibers, films and other shaped articles.
Further, elastomeric polyesters, inter alia, polytetramethylene terephthalate elastomers, are being utilized extensively in the uses for automotive parts, electric and electronic parts, etc., by virtue of excellence in oil resistance, chemical resistance, availability in a wide temperature range, load bearing properties and resistance to flexural fatigue as compared with other thermoplastic elastomers. Namely, the polytetramethylene terephthalate units constituting hard segments provide the elastomers with a high melting point and high crystallinity, while the polyalkylene glycol units constituting soft segments are extremely low in glass transition temperature. Consequently, the elastomers maintain flexibility and strength in a wide temperature range from a low temperature to a high temperature. Furthermore, their mechanical properties such as tenacity, elongation or the like and chemical properties such as oil resistance, chemical resistance or the like can be adjusted by varying proportion of the two units depending on the purposes and uses.
The polyalkylene terephthalates are usually obtained by a two stage reaction. On the first stage, a transesterification reaction of dimethyl terephthalate and an alkylene glycol is effected by heating at 150.degree..about.240.degree. C. in the presence of a catalyst, removing producing methanol, or a direct esterification reaction of terephthalic acid with an alkylene glycol is effected, to produce a precondensate, bis(hydroxyalkyl)terephthalate. On the second stage, the precondensate undergoes polycondensation with a deglycol reaction. In this case, the polycondensation is conducted at a temperature above the melting point of the polyalkylene terephthalate under a reduced pressure, or the precondensate is once cooled to solidify, pulverized into powder or particles of an appropriate size and then undergoes a solid phase polymerization at a temperature below the melting point of the polyalkylene terephthalate under a reduced pressure or an inert gas stream. In order to promote these reactions smoothly, catalysts have generally been employed and it has been known that not only the reaction rate but also the color shade and heat stability of the resultant polyesters largely depend upon the kind of catalysts employed.
Accordingly, it is extremely important to provide catalysts having good performances such as to yield, at a high reaction rate, polyesters having a good color shade and exhibiting an excellent heat stability. Therefore, a host of catalysts for polycondensation which are suitable for the manufacture of linear polyesters having a high polymerization degree have so far been proposed.
With respect to metallic niobium or niobium compounds catalysts, there are disclosed: in U.S. patent specification No. 3,127,375, a process for preparing a polyester using a niobium compound such as niobium pentoxide or the like as a catalyst for polycondensation; in U.S. patent specification No. 3,758,438, a process for the manufacture of fibers from polyesters having a metallic golden shade wherein niobium pentoxide hydrate having a water content from 5.about.25% is added during esterification and polycondensation or transesterification and polycondensation in the formation of polyesters; in U.S. Pat. No. 4,365,031, a process for producing polyester articles having a green shade wherein a yellow colorant, i.e., niobium compound, is employed as a catalyst for esterification and/or polycondensation and then a blue pigment is added; in Japanese Pat. Application Publication No. 8,225/1972, a process for polycondensation of polyesters by using metallic niobium as a catalyst; in Japanese Patent Application Publication No. 40,310/1970, a process wherein a completely hydrolyzed product of niobium pentahalide is used as a catalyst for polycondensation; in Japanese Patent Application Publication No. 12,153/1971, a process wherein an aromatic oxide of niobium is employed as a catalyst for polycondensation; in Japanese Patent Application Laid-open No. 56-20,027, a process for preparing polyethylene terephthalate wherein a trace of an ethylene glycol soluble niobium compound is incorporated into other catalysts for polycondensation; etc.
However, as a result of detailed studies, these hitherto proposed niobium catalysts have proved to be not always satisfactory in respect of the effect for reaction promotion or solubility in reaction system of the catalysts. Particularly in the case of polyethylene terephthalate, there is encountered a difficulty such that color development of the products is so manifest that the article value is impaired.
Furthermore, a process for manufacturing polytetramethylene terephthalate by direct polymerization of terephthalic acid with 1,4-tetramethylene glycol as well as a manufacturing process through transesterification of dimethyl terephthalate and 1,4-tetramethylene glycol, consists of a first stage reaction wherein terephthalic acid and 1,4-tetramethylene glycol are subjected to an esterification reaction to produce bis(4-hydroxybutyl)tetephthalate and its oligomers and a second stage reaction wherein these are further polycondensed. On the second stage of such a manufacturing process, in the absence of catalysts for polycondensation, the reaction is extremely retarded and viscosity of the condensates does not rise higher than a certain extent. Namely, in order to promote the polycondensation to attain a viscosity that is required for objective fibers or resins, catalysts such as organic titanium compounds, organic tin compounds or the like have generally been added.
Meanwhile as a manufacturing process of the above mentioned elastomeric polyesters, there is generally known a process wherein an esterification reaction of terephthalic acid, ethylene glycol or 1,4-tetramethylene glycol and a polyalkylene glycol is conducted, followed by a polycondensation reaction, or after the estrification reaction of the former two reactants, the polyalkylene glycol is incorporated to the reaction system and subsequently the polycondensation reaction is conducted.
In such a process, difficulties are also encountered that the polyalkylene glycol used as a material for the soft segment is so susceptible to thermal decomposition at high temperatures or the polymerization at low temperatures requires such a long time that it is hard to obtain polymers having an aimed viscosity. As measures therefor, there have been proposed a method of using organic titanium catalysts in Japanese Patent Application Publication No. 31,558/1974 and a method of combined use of organic titanium catalysts with aromatic aldehyde acid in Japanese Patent Application Publication No. 29,096/1980.
However, as a result of detailed studies of those methods of using organic titanium catalysts, it has been found that these catalysts have a drawback of being denatured and deactivated due to water present in the reaction system. Namely, they are not satisfactory because, on the one hand, if they are used in an amount sufficient to promote the reaction, a turbidity appears due to the denaturing and precipitating of the organic titanium so that opaque polymers are obtained and, on the other hand, if used in a reduced amount, the reaction time is so much prolonged that the decomposition of the polyalkylene glycol components increases. Further, it has also been found that the use of organic tin compounds is involved in a problem of color development of the resulting polymers.
Therefore, we, the inventors, as a result of an assiduous research to seek excellent catalysts for polycondensation of liner polyesters, have found that specified niobium compounds soluble in the reaction mixture can shorten the reaction time astonishingly and provide the resulting polymers with satisfactory properties, and thus accomplished the present invention.